Archive for June, 2010

The Squat Urchin Shrimp (Gnathophylloides mineri) is an amazingly successful creature that can be found living amongst the spines of sea urchins throughout most of the world’s shallow tropical waters. In the Caribbean they hitchhike exclusively upon the black and white West Indian Sea Egg (Tripneustes ventricosus), traveling along where ever its host may go. The squat urchin shrimp is very small, reaching no more than 6mm in length, and orients itself parallel with the spines making it all but invisible and protected from a would-be-predator. Often colonies of up to half a dozen squat urchin shrimp of varying sizes will all share the same urchin. Beyond its circumtropical distribution and perfect camouflage, the squat urchin shrimp further demonstrates its successfulness by feeding upon the epidermal tissue of the very spines that grant it protection. This is a relatively benign form of parasitism that doesn’t seem to bother the urchin. These shrimp will also feed opportunistically upon detritus that the urchin picks up as it moves along the sea floor. The squat urchin shrimp is a creature that has found a near perfect niche in a truly self-sustaining, self-contained world of spines.

Barely 7mm in size, the aptly named heart urchin pea crab (Dissodactylus primitivus) lives its entire life as a passenger upon the slow-moving red heart urchin (Meoma ventricosa). It is an example of the unusual life that can be found by looking in unexpected places on Floridian coral reefs. The red heart urchin is an unusual member of the echinoderm clan (e.g. urchins, sea stars, sand dollars, sea cucumbers) that spends most of its time burrowing in the sand. It sifts through the grains of sand searching for organic detritus that constitutes its diet. Likewise, the heart urchin pea crab lives a well-protected life (usually below the sand) amongst the spines of this fist-sized urchin. While most crabs move swiftly, this pea crab moves slowly in order to navigate through the corridors of spines, even spending time inside the urchin’s mouth. It is likely that the crab feeds upon some of the food that would otherwise be consumed by the urchin. This commensal relationship appears mildly parasitic, as the urchin doesn’t seem to gain any sort of direct benefit from the crab living amongst its spines. Frequently, several heart urchin pea crabs will live communally without any noticeable negative impact to their host urchin’s health.

If you look closely, you’ll notice the rhythmic working of its gills and circulatory system within the heart urchin pea crab’s translucent, eggshell exoskeleton.

The sun anemone shrimp (Periclimenes rathbunae) is the least common of the three species of Floridian anemone shrimp. While the other two anemone shrimp (P. pedersoni and P. yucatanicus) act as cleaners to passing fish, the sun anemone shrimp doesn’t seem to engage in this behavior. Instead, it spends its time living almost exclusively upon its namesake sun anemone (Stichodactyla helianthus). Aquarium observations suggest that this shrimp may supplement its diet by occasionally nipping off and eating the tentacles of the anemone. This parasitism suggests a more complicated symbiotic relationship than the sort of simple mutualism that these shrimp are often categorized by.

In Floridian waters, the scarcity of this shrimp is likely related to the infrequency of its host sun anemone. However, where they are found, the sun anemone often lives in dense clonal colonies that can literally carpet shallow reefs. The tentacles, while short and stubby, are packed with powerful stinging nematocysts that act like microscopic harpoons to deliver their venom. The end result of all these nematocysts and tentacles, is an anemone that is very ‘sticky’, and capable of producing painful welts to the careless diver.